Resin transfer molding with honeycomb core and core filler

ABSTRACT

A process for making a layered product having a honeycomb core having cells filled with a foam material, the process comprising the steps of: (a) placing a first layer of an uncured, heat-expandable, foamable material on the top side of a central honeycomb core having empty cells, and placing a second layer of an uncured, heat-expandable, foamable material on the bottom side of the central honeycomb core; (b) placing a first layer of an uncured preform material above the first layer of uncured, heat-expandable, foamable material, and placing a second layer of an uncured preform material above the second layer of uncured, heat-expandable, foamable material; (c) placing the charge made by steps (a) and (b) inside a mold and closing the mold; (d) heating the mold to the cure temperature of the heat-expandable, foamable material, and holding the mold at this temperature for sufficient time to expand and cure the heat-expandable, foamable material; (e) reducing the temperature of the mold to the injection temperature of a selected resin transfer molding (RTM) resin system, and injecting the selected resin transfer molding (RTM) resin system into the mold; (f) holding the temperature of the mold at the cure temperature for the resin transfer molding (RTM) resin system for sufficient time to cure the resin system; and, (g) removing the product from the mold after curing is completed.

CROSS-REFERENCE TO ANOTHER APPLICATION

This is a divisional of application Ser. No. 08/368,063 filed on Jan. 3,1995 now U.S. Pat. No. 5,569,508.

This is a cross-reference to co-pending U.S. patent application Ser. No.08/368,062, filed by U.S. Express Mail No. GB370067027US, filed on Jan.3, 1995 (the same date as the parent of this application), now U.S. Pat.No. 5,567,499 entitled "Resin Transfer Molding In Combination WithHoneycomb Core," invented by Thomas R. Cundiff and Bradley A. Frye,commonly-assigned to The Boeing Company, and having Attorney Docket No.92-275. The aforesaid patent application of Thomas R. Cundiff andBradley A. Frye is hereby incorporated by reference in this application.

BACKGROUND OF THE INVENTION

1) Field of the Invention

This invention relates to a process of resin transfer molding (RTM) usedin combination with honeycomb core material and a heat-expandable,foamable material which is heated and expanded to fill the cells of thehoneycomb core material. This invention also relates to the strong,lightweight products made thereby. More particularly, the process is formaking products through the use of resin transfer molding (RTM) whereinthe final product includes a unit of honeycomb core material havingcells filled with a foam material (i.e., the RTM resin has been excludedfrom the cells of the honeycomb core). The strong, lightweight productsmade by this process are useful in many applications, for example, asaircraft parts.

2) Description of the Background Art

Resin transfer molding (RTM) allows the economical manufacture of highquality composites. Dry fibers, which may have the form of continuousstrand mat, unidirectional, woven or knitted preforms, are placed in aclosed mold and resin is then introduced into the mold under externalpressure or vacuum. The resin cures under the action of its ownexotherm, or heat may be applied to the mold to complete the curingprocess. Early applications of the resin transfer molding techniqueemployed unsaturated polyester resins. Polyester and vinyl ester resinsare used in resin-transfer-molded consumer products, pipes, pressurevessels, and automotive applications. Epoxy resins have also beendeveloped for resin transfer molding of high quality, high fiber volumefraction composite components in electronic and aerospace applications.

Resin transfer molding is a process where the resin system istransferred at low viscosities and low pressures into a closed mold diecontaining a preform of dry fibers. The RTM process can be used toproduce low cost composite parts that are complex in shape. These partstypically require continuous fiber reinforcement along with inside moldline and outside mold line controlled surfaces. It is the placement ofcontinuous fiber reinforcements in large structures that sets RTM apartfrom other liquid molding processes.

For five decades, resin transfer molding has been used for applicationssuitable to consumer product markets. However, in the last decadethrough the development of high-strength resin systems and more advancedpumping systems, RTM has advanced to new levels. These recentdevelopments have promoted this technology as a practical manufacturingoption for high-strength composite designs, particularly in theaerospace industry.

The following are some of the fundamental advantages of the resintransfer molding process: (1) complex shapes (detail integration); (2)low part variability (product of the mold); (3) good surface finish; (4)55 to 70% by weight fiber/resin ratio control; (5) eliminates autoclavecycle; (6) low material costs; (7) minimal training costs; (8) lowcapital investment costs; (9) low worker exposure; and, (10) bushingsand inserts can be molded in.

In the aerospace industry, the most visible advantage to this moldingprocess lies in its ability to make complex shapes, that is, to combinemultiple, detailed components into one configuration. For example, manytraditional designs consist of many individual details that are combinedas a subassembly. These subassemblies usually require labor-intensiveshimming, bonding, mechanical fastening, and sealing. Consequently,these subassemblies demonstrate high part-to-part variability due totolerance buildup.

Individual components are integrated into one item with resin transfermolding. Therefore, the part-to-part variation is low because the partsare a product of the mold.

Aerodynamic, decorative finish and controlled fit-up surfaces aretypical part characteristics in the aerospace industry. Thesehigh-quality surface-finish characteristics are ideal for RTM.Therefore, being a product of the mold makes the surface quality of thepart comparable to that of the tool's surface.

Another advantage of RTM is the control of the reinforcement/resinratio, which is typically 55 to 70% fiber by weight. This produces partsthat are lightweight and high in strength.

Because the method of heat transfer is integrated into the mold die, theneed for an autoclave is eliminated. Therefore, no autoclave costs areincurred, no size limitations are inherent, and no staging issues occur.

In terms of raw material costs, RTM offers cost savings by using bulkmaterials like broad goods. Because dry goods are less expensive thanpreimpregnated materials, a savings can be associated with the cost ofthe wasted material during the ply-knitting operation. Also, bulkmaterials do not need special handling requirements such as freezerstorage.

The basic injection operation of RTM is straight-forward and easilylearned. Hence, minimal training is required to bring operators on line.On the other hand, in making preforms the level of operator skill andtraining is dependent on the method of preforming that is used.

The initial capital investment costs of RTM are low when compared withthe many other molding processes. The most elementary approach to RTMcan be achieved using a pressure pot, an oven, and a vacuum source. Avariety of commercially-available equipment can be used to advance theprocess in many areas.

In most cases, RTM materials can be used with minimal chemical exposureto workers and their environment. Many high-performance resin systemsare stable and release low volatiles. Since RTM is processed within aclosed system, workers are exposed to the resin only when loading thedispensing equipment.

Bushings and inserts can be incorporated into the preform and injectedin-place to eliminate some higher level assembly. Specialconsiderations, however, must be made in the design and fabrication ofthe mold die (i.e., value added vs. tool cost).

Some of the limitations of RTM include: (1) higher tool cost; (2) designchanges can be costly (tooling costs); (3) cost of advanced preformingarchitecture; (4) cost of custom resin systems; and, (5) tool handlingchallenges (size and weight of tools).

Due to the high quality of the mold and inherent complexity, tooling isexpensive. Parts with complex configurations have costly multi-piece,break-down tooling.

Design changes can be costly when modifying complex multi-piece molds.Even a simple design change can result in extensive rework or toolremake.

The cost of advance preforming architecture can be high due to slowlabor-intensive processes.

The resin systems must meet design and process parameters that may bedifficult to combine. For example, design criteria such as mechanicaltest values or flammability values must coincide with the processcriteria such as pot life, viscosity, worker exposure, and cure time.Resin tougheners, in general, cannot be added because the preform actsas a prime filter entrapping these materials at the point of induction.

One of the benefits of RTM is the ability to manufacture large parts.However, it can also be a major limitation because the tools are largeand heavy. Large and massive molds have special handling needs that caninclude cranes, trunnions, and fork lifts.

The special problem involved in using resin transfer molding to makeproducts that include a unit of honeycomb core material is the exclusionof the resin from the cells of the honeycomb core material. If thehoneycomb core cells are not isolated from the resin being injected intothe mold, the cells of the honeycomb core will fill with resin and avery heavy product will be the result.

SUMMARY OF THE INVENTION

The present invention solves the foregoing problem and provides a meansof making strong, lightweight products that include units of honeycombcore material having cells filled with a foam material (i.e., the RTMresin has been excluded from the cells of the honeycomb core). The cellsof the honeycomb core material are isolated from the RTM resin by thefoam material. The foam material provides a barrier to the RTM resinsystem so that when the resin system is injected into the mold the resinsystem is excluded from the cells of the honeycomb core material.

In one aspect, the present invention is a layered product made in a moldand having a honeycomb core having cells filled with a foam material,the product having: (a) a central honeycomb core having cells filledwith a foam material, the foam material having been cured inside themold; and, (b) a first layer of a cured preform impregnated with a resintransfer molding (RTM) resin system above the central honeycomb core,and a second layer of a cured preform impregnated with a resin transfermolding (RTM) resin system below the central honeycomb core, the firstand second layers of preform impregnated with a resin transfer molding(RTM) resin system having been cured inside the mold.

In another aspect, the present invention is a process for making alayered product having a honeycomb core having cells filled with a foammaterial, the process comprising the steps of: (a) placing a first layerof an uncured, heat-expandable, foamable material on the top side of acentral honeycomb core having empty cells, and placing a second layer ofan uncured, heat-expandable, foamable material on the bottom side of thecentral honeycomb core; (b) placing a first layer of an uncured preformmaterial above the first layer of uncured, heat-expandable, foamablematerial, and placing a second layer of an uncured preform materialabove the second layer of uncured, heat-expandable, foamable material;(c) placing the charge made by steps (a) and (b) inside a mold andclosing the mold; (d) heating the mold to the cure temperature of theheat-expandable, foamable material, and holding the mold at thistemperature for sufficient time to expand and cure the heat-expandable,foamable material; (e) reducing the temperature of the mold to theinjection temperature of a selected resin transfer molding (RTM) resinsystem, and injecting the selected resin transfer molding (RTM) resinsystem into the mold; (f) holding the temperature of the mold at thecure temperature for the resin transfer molding (RTM) resin system forsufficient time to cure the resin system; and, (g) removing the productfrom the mold after curing is completed.

In yet another aspect, the invention is a layered aircraft part made ina mold and having a honeycomb core having cells filled with a foammaterial, the aircraft part having: (a) a central honeycomb core havingcells filled with a foam material, the foam material having been curedinside the mold; and, (b) a first layer of a cured preform impregnatedwith a resin transfer molding (RTM) resin system above the centralhoneycomb core, and a second layer of a cured preform impregnated with aresin transfer molding (RTM) resin system below the central honeycombcore, the first and second layers of preform impregnated with a resintransfer molding (RTM) resin system having been cured inside the mold.

Finally, in yet another aspect, the invention is a process for making alayered aircraft part having a honeycomb core having cells filled with afoam material, the process comprising the steps of: (a) placing a layerof an uncured, heat-expandable, foamable material on the top side of acentral honeycomb core having empty cells, and placing a second layer ofan uncured, heat-expandable, foamable material on the bottom side of thecentral honeycomb core; (b) placing a first layer of an uncured preformmaterial above the first layer of uncured, heat-expandable, foamablematerial, and placing a second layer of an uncured preform materialabove the second layer of uncured, heat-expandable, foamable material;(c) placing the charge made by steps (a) and (b) inside a mold andclosing the mold; (d) heating the mold to the cure temperature of theheat-expandable, foamable material, and holding the mold at thistemperature for sufficient time to expand and cure the heat-expandable,foamable material; (e) reducing the temperature of the mold to theinjection temperature of a selected resin transfer molding (RTM) resinsystem, and injecting the selected resin transfer molding (RTM) resinsystem into the mold; (f) holding the temperature of the mold at thecure temperature for the resin transfer molding (RTM) resin system forsufficient time to cure the resin system; and, (g) removing the partfrom the mold after curing is completed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show how the samemay be carried into effect, reference will now be made, by way ofexample, to the accompanying drawings. In the different figures,elements designated by like reference numerals have correspondingfunctions.

FIG. 1 is a schematic perspective view, partially in cutaway, of thefinal cured product of the invention showing the layers above and belowthe central honeycomb core material and showing the cells of thehoneycomb core material filled with a cured foam material.

FIG. 2 is a schematic vertical sectional view of the uncured lay-up ofthe product of the invention showing the layers above and below thecentral honeycomb core material and showing (at this point in theprocess) the empty cells of the honeycomb core material.

FIG. 3 is a schematic vertical sectional view, taken along line 3--3 ofFIG. 1, of the final cured product of the invention showing the layersabove and below the central honeycomb core material and showing thecells of the honeycomb core material filled with a cured foam material.

FIG. 4 is a schematic vertical sectional view of the die mold used inresin transfer molding (RTM) to make the final cured product of theinvention shown in FIGS. 1 and 3.

FIG. 5 is a schematic perspective view, partially in cutaway, of analternative embodiment of the final cured product of the inventionshowing the layers above and below the central honeycomb core materialand showing the cells of the honeycomb core material filled with a curedfoam material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To design and fabricate parts using the RTM process, one must evaluatethe part requirements based on the following: (1) the part requireshigh-strength, continuous-fiber reinforcement with 55 to 75% fiber byweight; (2) the part is complex in nature with designed-in flanges,stiffeners, or inserts; (3) the part tolerance requirements typicallyrange from ±0.10 to ±0.30; (4) the part thickness ranges from 0.040 inchup to 5.0 inches; and, (5) the surface finish of the part must be 125RMS or better (well within typical machining parameters).

Some of the most common materials used to fabricate dry fiber preformsare fiberglass, graphite, aramid, and ceramics. These fiber materialscan be used separately or combined into a variety of hybrids to meetspecific performance needs. Since these materials have been used incomposite manufacturing for many years, they offer enhanced designflexibility. There are several ways to form the reinforcements to adesired shape. Different preform styles include braiding, knitting,weaving, filament winding, and stitching. Each of these styles isinherently unique and must be individually evaluated for specific designcharacteristics. Woven and filament wound preforms typically require abinding agent to maintain consolidation and configuration. Bindingagents work best if they are a derivative of the neat resin system. Insome situations, it is necessary to use a binder that is dissimilar. Ifthis is the case, the binder can migrate, consolidating during injectionin an uncontrolled manner, and reducing the strength of the base resinsystem.

All dry fiber preforms should be consolidated prior to loading the mold.Consolidated preforms ensure appropriate fiber orientation and volume.Fiber orientation and distortion in the RTM process must be understoodfrom two aspects. The first deals with the positioning, forming, andconsolidating of dry fibers. Unlike preimpregnated reinforcements, dryfibers are unstable and are easily distorted. Binders help to stabilizethe fibers during their handling and positioning. During the forming andconsolidation process, the preform becomes stable. Once the preform isconsolidated, it can be handled, assembled, and located in the mold withlimited distortion. The second aspect deals with the ability to hold andlock the correct fiber orientation during resin injection. When thepreform is closed in the mold, it is further consolidated by the closingpressure. The preform's high fiber content locks the fibers tightly inplace, preventing fiber movement.

The quality of the mold is most critical to the RTM process. The surfacefinish and dimensional control are in the products of the mold. In otherwords, the time and money spent to make high quality molds will yieldhigh quality parts.

When selecting an RTM resin system for design, the first step is toclearly define the performance conditions. Some of the performancecriteria includes the range of operating temperatures, thermal cycles,and mechanical properties. To ensure the proper resin selection, theresin properties must be evaluated based on the performance conditions.A wide variety of RTM resin systems is available for use in the presentinvention, along with many others that are in the development stage.Some of the generic RTM resin systems that can be used include: epoxyresin systems; cyanate ester resin systems; vinyl ester resin systems,phenolic resin systems, polyester resin systems, and bismaleimide resinsystems.

The list of resin characteristics serves as the base to define the resintransfer process parameters. Resins that are conducive to the RTMprocess demonstrate low viscosity (ideally 500 cps or less), extendedpot life, low volatile content, and low exothermic temperatures with asemi-rapid gel time.

RTM resin systems are available in different formulations such asone-part or two-part systems. Cleanup operations of non-cross-linkedformulations require solvents. Therefore, solvent cleanup should beconsidered when selecting an RTM resin system, although appropriatepumping equipment can store, load, and pump many of these resin systemsdirectly from their shipping containers and require minimal solventcleanup. Solvents are discouraged because they increase both workerexposure and hazardous waste disposal.

There are several types of RTM resin delivery systems available on thecommercial market that can be employed in the present invention. Thepump mechanism can be powered with one or a combination of pneumatic,hydraulic, or gear drive systems. The resin injection operation is keyto successfully carrying out the process. Better results have beenexperienced using positive displacement pumps. A positive displacementpump is preferred when the configuration of the part is large orcomplex. This type of pump provides constant pressure and continuousresin flow. At the same time, these systems provide the means to controland optimize the injection cycle.

The most elementary pumping system is a pneumatic pressure pot. Thistype of system has produced many successful parts with RTM. However, itis limited by the degree of control over the rate and pressures of theresin flow front. On the other hand, advanced resin delivery systemsprovide positive displacement combined with computer control featurescontrolling critical variables of the resin injection operation that areotherwise operator sensitive.

The type of resin being used is important when selecting a resindelivery system. Many of the delivery systems offer an array of optionsthat the user must evaluate. For example, the generic differencesbetween a two-part and a one-part resin system can illustrate this. Atwo-part resin involves a pump with features that offer metering,mixing, and delivering. A one-part system does not require mixing, sothe pump is used for delivery only.

Many resin delivery systems offer enhanced features that improve theprocess such as an ability both to maintain a predetermined hydrostaticresin pressure and to adjust and display the temperature for viscositycontrol as well as for resin flow rate and volume control. These systemsare generally simple to set up and operate, and are easy to clean andmaintain.

Because there are many variables that influence the RTM process, thefollowing are some preferred guidelines to the RTM process: (1) fiberloading for structural applications at 55-65% by weight; (2) hard vacuumassistance provides better resin flow for complete ply wet out; (3)resin viscosity less than 500 cps allows lower injection pressure; (4)preconsolidated preform is complete, ready for mold loading; (5) mold isintegrally heated to reduce cycle time and mold handling; (6) resin ispreviously degassed to minimize porosity and void content; (7)hydrostatic pressure is held after resin injection to lower porositycontent; and, (8) injection pressure is less than 100 psi to allow aslow-moving flow front with minimal fiber distortion.

The problem involved in using resin transfer molding to make productsthat include a unit of honeycomb core material is the exclusion of theRTM resin from the cells of the honeycomb core material. The presentinvention solves this problem and provides a process for making strong,lightweight products that include units of honeycomb core materialhaving cells filled with a cured foam material (i.e., the RTM resin hasbeen excluded from the cells of the honeycomb core). The cells of thehoneycomb core material are isolated on both sides by the cured foammaterial. The cured foam material provides a barrier for the cells ofthe honeycomb core material prior to injection of the RTM resin systemso that the resin system is excluded from the cells of the honeycombcore material.

The Product Of The Invention

Referring to the drawings, FIGS. 1 and 3 show schematically the layeredconstruction of the final product 40 of the invention. The final product40 is constructed of a central honeycomb core material 12 having cellsfilled with a cured foam material 46 (i.e., the RTM resin has beenexcluded from the cells of the honeycomb core 12) and cured layers 18a,18b (shown schematically) comprised of the preform fibers which havebeen impregnated with the RTM resin system and cured inside the mold 22(FIG. 4).

FIG. 2 shows schematically the layered construction of the uncuredlay-up of the product of the invention showing the central honeycombcore material 12 Having empty cells (at this point in the process beforethe heat-expandable, foamable material 44a, 44b has been heated andexpanded); the layers of an uncured heat-expandable, foamable material44a, 44b; and the layers of a preform 24a, 24b of dry fibers (before theRTM resin system has been injected into the mold).

FIG. 4 shows schematically a typical RTM heated die mold 22 having a lid22a and having upper and lower inlet ports 20a and 20b where the RTMresin system is injected into the closed mold. The charge inside mold 22is constructed of the central honeycomb core material 12 having emptycells (at this point in the process before the heat-expandable, foamablematerial 44a, 44b has been heated and expanded); layers of an uncuredheat-expandable, foamable material 44a, 44b; and layers of a preform24a, 24b of dry fibers (before the RTM resin system has been injectedinto the mold).

The honeycomb core material can be any of the honeycomb core materialssuch as: aluminum, aluminum alloy, fiberglass, NOMEX, and carboncomposite. The preform dry fibers can be any of the fibers describedabove such as: fiberglass, carbon (graphite), aramid, and ceramics. Theprepreg material can be any of the prepreg materials. The primaryrequirement is that the cure temperature of the prepreg material shouldbe the same as the cure temperature of the adhesive film. The RTM resinsystem can be any of the conventional RTM resin systems described abovesuch as: epoxy resin systems; cyanate ester resin systems; vinyl esterresin systems, phenolic resin systems, polyester resin systems, andbismaleimide resin systems.

The preferred material that may be employed as the heat-expandable,foamable material 44a, 44b (FIGS. 2 and 4) is SYNSPAND X9899. Theuncured preferred material is described as follows:

Supplier: Dexter Aerospace, Materials Division; Pittsburg, Calif.Product Designation: SYNSPAND X9899 Composition: Component (1)--Adhesivemixture containing bisphenol A/epichlorohydrin epoxy resin 30-60%by wt.;and Component (2)--Inert carriers 30-60% by wt. The Process Of TheInvention

The process of the invention is as follows. The first two steps ofbuilding up the charge (or sandwich) can be accomplished inside the mold22 (FIG. 4) before it is closed or the steps can be accomplished outsidethe mold.

The first step is to place a layer of the uncured heat-expandable,foamable material 44a, 44b on the top, bottom, and vertical sides of thehoneycomb core material 12 (which at this time has empty cells).

The second step is to place the layers 24a, 24b of dry fiber preformover the top layer 44a of the uncured heat-expandable, foamable materialand under the bottom layer of the uncured heat-expandable foamablematerial 44b. This completes the charge.

The third step is to place the charge inside the mold 22 and to closethe mold.

The fourth step is to heat the mold to the cure temperature of theheat-expandable, foamable material 44a, 44b and to hold the mold at thistemperature for sufficient time to expand and cure the heat-expandable,foamable material. The cure temperature can vary depending on thespecific heat-expandable, foamable material being employed. For example,for SYNSPAND X9899, the cure temperature is about 350° F. The cure timecan vary depending on the specific foamable material being employed. Forexample, for SYNSPAND X9899, the cure time is about 90 minutes. At thispoint in the process, there is no pressure added inside the mold beyondthe mechanical pressure of the closed mold pressing against the chargeand consolidating the charge. During the cure, the heat-expandable,foamable material changes its physical state to a foam that expands andfills the cells of the honeycomb core. The net effect is that the layerof heat-expandable, foamable material transforms entirely into a foam.The upper layer 44a of heat-expandable, foamable material generates afoam that expands downwardly into the cells of the honeycomb core belowit. The lower layer 44b of heat-expandable, foamable material generatesa foam that expands upwardly into the cells of the honeycomb core aboveit. The two advancing fronts of foam meet near the mid-plane of each ofthe cells, thereby creating a boundary line 48 (FIGS. 1 and 3) in eachcell where the advancing fronts of foam met. The density of theresulting cured foam is in the range of 10-40 lbs/ft³. A low densitycured foam is preferred for aircraft parts where low weight is the goal.The layers 24a, 24b of dry fiber preform (FIGS. 2 and 3) are notimpregnated by the foam during the foregoing expansion and cure of thelayers of heat-expandable, foamable material.

The fifth step is to reduce the temperature of the mold 22 to the RTMresin system injection temperature and to inject the RTM resin systeminto the mold through the inlet ports 20a, 20b. The cured foam 46 willnot allow the RTM resin into the cells of the honeycomb core 12. The RTMresin system injection temperature can range from 250° F. to 350° F.depending on the specific RTM resin system being employed. An example ofa one-part epoxy RTM resin system is the product PR 500 made by 3MCompany, Aerospace Materials Department, St. Paul, Minn., which has abuilt-in curing catalyst that is activated at the cure temperature ofabout 320° F. At this point in the process, after the mold is full ofRTM resin, a hydrostatic pressure may be applied to fill any voids thathave not been filled by the resin. The pressure is applied through thepumping system. For example, the pressure applied by the pumping systemmay be in the range of 20-60 psi.

The seventh step is to increase (if necessary) the temperature of themold to the cure temperature for the RTM resin system and to hold themold at this temperature for sufficient time to cure the RTM resinsystem. Again, the RTM resin system injection temperature can range from250° F. to 350° F. depending on the specific RTM resin system beingemployed. After curing is complete, the part is removed from the mold.

An Alternative Embodiment Of The Invention

FIG. 5 is a schematic perspective view, partially in cutaway, of analternative final cured product 50. Reference is made to co-pending U.S.patent application Ser. No. 08/368,062, filed by U.S. Express Mail No.GB370067027US, filed on Jan. 3, 1995 (the same date as the parent ofthis application), now U.S. Pat. No. 5,567,499 entitled "Resin TransferMolding In Combination With Honeycomb Core," invented by Thomas R.Cundiff and Bradley A. Frye, commonly-assigned to The Boeing Company,and having Attorney Docket No. 92-275. The aforesaid patent applicationof Thomas R. Cundiff and Bradley A. Frye is hereby incorporated byreference in this application. Final product 50 (FIG. 5) is acombination of the invention of the present application and theinvention of the aforesaid patent application of Thomas R. Cundiff andBradley A. Frye.

Referring to FIG. 5, the alternative final product 50 is constructed ofa central honeycomb core material 12 having cells filled with a curedfoam material 46 (i.e., the RTM resin has been excluded from the cellsof the honeycomb core 12); layers of a cured adhesive film 14a, 14bwhich have been cured inside the mold; layers of a cured prepregmaterial 16a, 16b which have been cured inside the mold, and curedlayers 18a, 18b (shown schematically) comprised of the preform fiberswhich have been impregnated with the RTM resin system and cured insidethe mold.

The process of making the alternative final product 50 is a combinationof the process described in the present application and the processdescribed in the aforesaid patent application of Thomas R. Cundiff andBradley A. Frye. The process is as follows. The first three steps ofbuilding up the charge (or sandwich) can be accomplished inside the moldbefore it is closed or the steps can be accomplished outside the mold.

The first step is to place a layer of the uncured heat-expandable,foamable material on the top, bottom, and vertical sides of thehoneycomb core material 12 (which at this time has empty cells).

The second step is to place a layer of the adhesive film 14a, 14b on thetop side and bottom side of the honeycomb core material 12.

The third step is to place a layer of the prepreg material 16a, 16b overeach layer of the adhesive film 14a, 14b.

The fourth step is to place the dry fiber preform 24a, 24b over eachlayer of the prepreg material 16a, 16b. This completes the charge.

The fifth step is to place the charge inside the mold 22 and to closethe mold.

The sixth step is to heat the mold to the cure temperature of theheat-expandable, foamable material, the adhesive film 14a, 14b, and theprepreg material 16a, 16b and to hold the mold at this temperature forsufficient time to expand and cure the heat-expandable, foamablematerial, to cure the adhesive film, and to cure the prepreg.

The seventh step is to reduce the temperature of the mold to the RTMresin system injection temperature and to inject the RTM resin systeminto the mold through the inlet ports. At this point in the process,after the mold is full of resin, a hydrostatic pressure may be appliedto fill any voids that have not be filled by the resin. The pressure isapplied through the pumping system. For example, the pressure applied bythe pumping system may be in the range of 20-60 psi.

The eighth step is to increase (if necessary) the temperature of themold to the cure temperature for the RTM resin system and to hold themold at this temperature for sufficient time to cure the RTM resinsystem. After curing is complete, the part is removed from the mold.

As will be apparent to those skilled in the art to which the inventionis addressed, the present invention may be embodied in forms other thanthose specifically disclosed above, without departing from the spirit oressential characteristics of the invention. The particular embodimentsof the invention described above and the particular details of theprocesses described are therefore to be considered in all respects asillustrative and not restrictive. The scope of the present invention isas set forth in the appended claims rather than being limited to theexamples set forth in the foregoing description. Any and all equivalentsare intended to be embraced by the claims.

What is claimed is:
 1. A process for making a layered product having ahoneycomb core having cells filled with a foam material, said processcomprising the steps of:(a) placing a first layer of an uncuredheat-expandable foamable material on the top side of a central honeycombcore having empty cells, and placing a second layer of an uncuredheat-expandable foamable material on the bottom side of said centralhoneycomb core; (b) placing a first layer of an uncured preform materialdistal to said central honeycomb core and above said first layer ofuncured heat-expandable foamable material, and placing a second layer ofan uncured preform material distal to said central honeycomb core andbelow said second layer of uncured heat-expandable foamable material;(c) placing the charge made by steps (a) and (b) inside a mold andclosing the mold; (d) heating said mold to the cure temperature of saidheat-expandable foamable material, and holding said mold at thistemperature for sufficient time to expand and cure said heat-expandablefoamable material; (e) reducing the temperature of said mold to theinjection temperature of a selected resin transfer molding (RTM) resinsystem, and injecting said selected resin transfer molding (RTM) resinsystem into said mold; (f) holding the temperature of said mold at thecure temperature for said resin transfer molding (RTM) resin system forsufficient time to cure said resin system; and, (g) removing the productfrom said mold after curing is completed.
 2. A process for making alayered aircraft part having a honeycomb core having cells filled with afoam material, said process comprising the steps of:(a) placing a firstlayer of an uncured heat-expandable foamable material on the top side ofa central honeycomb core having empty cells, and placing a second layerof an uncured heat-expandable foamable material on the bottom side ofsaid central honeycomb core; (b) placing a first layer of an uncuredpreform material distal to said central honeycomb core and above saidfirst layer of uncured heat-expandable foamable material, and placing asecond layer of an uncured preform material distal to said centralhoneycomb core and below said second layer of uncured heat-expandablefoamable material; (c) placing the charge made by steps (a) and (b)inside a mold and closing the mold; (d) heating said mold to the curetemperature of said heat-expandable foamable material, and holding saidmold at this temperature for sufficient time to expand and cure saidheat-expandable, foamable material; (e) reducing the temperature of saidmold to the injection temperature of a selected resin transfer molding(RTM) resin system, and injecting said selected resin transfer molding(RTM) resin system into said mold; (f) holding the temperature of saidmold at the cure temperature for said resin transfer molding (RTM) resinsystem for sufficient time to cure said resin system; and, (g) removingthe part from said mold after curing is completed.
 3. A process formaking a layered product having a honeycomb core having cells filledwith a foam material, said process comprising the steps of:(a) placing afirst layer of an uncured heat-expandable foamable material on, the topside of a central honeycomb core having empty cells, and placing asecond layer of an uncured heat-expandable foamable material on thebottom side of said central honeycomb core; (b) placing a first layer ofan uncured adhesive film above said first layer of uncuredheat-expandable foamable material, and placing a second layer of anuncured adhesive film below said second layer of uncured heat-expandablefoamable material; (c) placing a first layer of an uncured prepregmaterial distal to said central honeycomb core and above said firstlayer of uncured adhesive film, and placing a second layer of an uncuredprepreg material distal to said central honeycomb core and below saidsecond layer of uncured adhesive film; (d) placing a first layer of adry fiber preform distal to said central honeycomb core and above saidfirst layer of uncured prepreg material, and placing a second layer of adry fiber preform distal to said central honeycomb core and below saidsecond layer of uncured prepreg material; (e) placing the charge made bysteps (a)-(d) inside a mold, and closing said mold; (f) heating saidmold to the cure temperature of said heat-expandable foamable material,said adhesive film, and said prepreg material, and holding said mold atthis temperature for sufficient time to cure said heat-expandablefoamable material, said adhesive film, and said prepreg material; (g)reducing the temperature of said mold to the injection temperature of aselected resin transfer molding (RTM) resin system, and injecting saidselected resin transfer molding (RTM) resin system into said mold; (h)holding the temperature of said mold at the cure temperature of saidresin transfer molding (RTM) resin system for sufficient time to curesaid resin system; and, (i) removing said product from said mold aftercuring is completed.